Cofounder Neil Capel tells us investors were excited by Sailthru's 30% month over month revenue growth. The startup has been cash-flow positive since April.

Sailthru began as an email service provider in 2008; it has since launched behaviorally-targeted tools used by more than 80 publishers and e-commerce sites.

Sailthru uses JavaScript to track everything users do on partner websites, from articles they read to items they view. Sailthru uses the information to personalize and recommend content, both on the sites and in email newsletters, which keep users engaged.

For instance, if a woman is looking at a black purse on Gilt Groupe, Sailthru might recommend another black bag for her to view next. And, If you go to Business Insider - Your News, you can see which articles Sailthru is recommending for you based on your recent reads.

"I wanted to build a system that solved the problems I had as a CTO," says Capel. "Email service providers only make it easy to send mass mail, not to communicate with individuals. Sailthru elongates the lifetime of every user on a website."

Capel says Sailthru will use the financing to double its staff from 30 to 60 people over the next twelve months. It will also be launching a behaviorally-targeted advertising product. It previously received $1 million in financing from DFJ Gotham in 2010.

Disclosure: Ian White, cofounder of Sailthru, was an early Business Insider employee and we're happy for him!